This application is a continuation of application Ser. No. 10/003,487, filed Nov. 1, 2001, now U.S. Pat. No. 6,591,109 and claims the benefit of provisional application 60/313,336 filed on Aug. 17, 2001.
The invention generally relates to wireless time division duplex (TDD) communication systems using code division multiple access (CDMA). In particular, the invention relates to reducing cross cell user equipment interference in such systems.
FIG. 1 is an illustration of a wireless TDD/CDMA communication system 10. The communication system 10 has base stations 12l to 12n (12) which communicate with user equipments (UEs) 14l to 14n (14). Each base station 12 has an associated operational area or cell. The base station communicates with UEs 14 in its cell.
In CDMA communication systems, multiple communications are sent over the same frequency spectrum. These communications are distinguished by their channelization codes. To more efficiently use the frequency spectrum, TDD/CDMA communication systems use repeating frames divided into timeslots, such as fifteen timeslots, for communication. In TDD, each cells' timeslots are used solely for either the uplink or downlink at a time. A communication sent in such a system has one or multiple associated code(s) or timeslot(s) assigned to it. The use of one code in one timeslot with spreading factor of sixteen is referred to as a resource unit.
Cross cell interference is a problem in such systems as illustrated in FIG. 2. If two different cells' UEs 14 are close to each other, their uplink transmissions interfere with the other UE's downlink transmissions in the same timeslot. As shown in FIG. 2, UE 141 uplink transmission U1 interferes with UE 142 downlink transmission D2. Likewise, UE 142 uplink transmission U2 interferes with UE 141 downlink transmission D1. Although the effective isotropic radiant power (EIRP) of UEs 14 is much less that base stations 12, the close proximity of the UEs 14 results in the unacceptable interference. This problem is exacerbated when adding new users or user services. Although a cell's base station and UE 14 may make timeslot interference measurements, such as interference signal code power (ISCP), to assure its new transmissions will not see unacceptable interference, other cells' users may end up experiencing unacceptable interference due to the new transmission. As a result, existing calls may be dropped or unacceptable quality of service (QOS) may occur.
Accordingly, it is desirable to reduce cross cell interference.